1. Field of the Invention
The present invention relates to rare earth-iron-born alloy magnet powders with improved magnetic properties, and to a process of producing the same.
2. Prior Art
Rare earth-iron-boron alloy magnet powders, comprising iron (Fe), boron (B) and a rare earth element inclusive of yttrium (Y) (which will be hereinafter represented by R), have been developed mainly for use as bonded magnets since rare earth-iron-boron alloys attracted attention as permanent magnet materials having superior magnetic properties. The bonded magnet is inferior in magnetic properties to the magnet powder contained therein or to other sintered magnets of the same kind, but is superior in physical strength and has such a high degree of freedom that it can be formed freely into an arbitrary shape, thereby varying application rapidly in recent years. Such bonded magnet is comprised of magnet powder bonded with organic or metal binders or the like, and its magnetic properties are influenced by those of the magnet powder.
In the alloy magnet powders as described above, their magnetic properties depend greatly on the structures of the alloy magnet powders, and hence research has been directed toward magnet powders with structures which make the best use of such superior magnetic properties of the alloys.
The rare earth-iron-boron alloy magnet powders hitherto known have been produced by various methods. (1) Japanese Patent Application A-Publication Nos. 59-219904, 60-257107 and 62-23903 describe a method of producing magnet powder which comprises crushing ingots, coarse powder or permanent magnets of the rare earth-iron-boron alloy by means of various mechanical crushing methods or a decrepitation or disintegration method involving hydrogenation-dehydrogenation.
FIG. 1(a) of the accompanying drawings schematically depicts one particle of rare earth-iron-boron alloy coarse powder which comprises a R.sub.2 Fe.sub.14 B intermetallic phase 1, a R-rich phase 2 and a B-rich phase 3, the R.sub.2 Fe.sub.14 B phase 1 serving as a principal phase. The coarse powder is crushed into fine powder, R.sub.2 Fe.sub.14 B phase 1 of which is subjected to transgranular or intergranular fracture, as shown in FIG. 1(b). Ingots or permanent magnets could as well be utilized instead of the coarse powder.
The alloy magnet powder crushed in this way keeps the structure of coarse powder, ingots or permanent magnets unchanged, and R.sub.2 Fe.sub.14 B phase 1 of each individual powder particle may be monocrystal or polycrystal depending upon the degree of crushing. For practical use, the magnet powder should have an average particle size ranging from several micrometers to several hundred micrometers, and its R.sub.2 Fe.sub.14 B phase has an average crystal grain size of 3 micrometers to several ten micrometers.
(2) Japanese Patent Application A-Publication Nos. 61-266502, 61-179801 and 61-214505 disclose the step of subjecting the magnet powder obtained according to the above method (1) to heat treatment to relieve strain or a further step of heating the powder at 800.degree. C. to 1,100.degree. C. to produce powder aggregates, in order to improve the coercivities. R.sub.2 Fe.sub.14 B phase of each individual particle of the powder is also kept unchanged during such treatment.
(3) Japanese Patent Application A-Publication Nos. 60-17905 and 60-207302 describe a method of producing rare earth-iron-boron alloy magnet powder which comprises the ste quenching a molten alloy by means of rapid quenching or atomizing to produce magnet powder. The magnet powder thus obtained may be subjected to heat treatment to improve the coercivities as occasion demands.
FIG. 2 schematically depicts one particle of the rare earth-iron-boron alloy magnet powder obtained by quenching a molten alloy. The powder particle has a polycrystalline structure of R.sub.2 Fe.sub.14 B phase 1, and there exist in its grain boundaries R-rich amorphous phase 2' surrounding the R.sub.2 Fe.sub.14 B phase 1. Such magnet powder has an average particle size of several micrometers to several hundred micrometers. The average crystal grain size of the R.sub.2 Fe.sub.14 B phase is of the order of several ten nanometers when the rapid quenching method is applied but is of the order of several ten micrometers in the case of the atomizing method.
The structure of the magnet powder thus produced is the one formed by solidification of the quenched molten alloy, or the one obtained by nucleation and growth of R.sub.2 Fe.sub.14 B phase through heat treatment at need. Therefore, the crystal orientations of the crystal grains in R.sub.2 Fe.sub.14 B phase are arbitrary, and the easy axes of magnetization of the magnetocrystalline anisotropy can be shown by the arrows designated at A in FIG. 2. Accordingly, each powder particle is not crystal anisotropic but isotropic, and hence is isotropic in its magnetic properties.
Other methods such as coreduction method and vapor phase method could as well be practiced to obtain rare earth-iron-boron alloy magnet powders, but the powders obtained by such method have structures similar to those of the powders produced by the aforementioned methods.
As described above, the prior art alloy powder has been such that its structure is defined by the structure of the ingots, coarse powder or permanent magnets kept unchanged, the one formed by solidification of quenched alloy melt, or the one obtained by heat treatment of such solidified structure.
Generally, it is assumed that in order to exhibit superior magnetic properties, the structure of the rare earth-iron boron magnet powder should satisfy the following conditions:
(i) R.sub.2 Fe.sub.14 B phase serving as the principal phase has an average crystal grain size of no greater than 50 .mu.m, preferably no greater than 0.3 .mu.m, wherein the crystal grains can be particles of a single magnetic domain. (ii) The principal phase has in its grains or at the grain boundaries neither impurities nor strain which may serve as nuclei upon the generation of reverse magnetic domain.
(iii) There exists R-rich phase or R-rich amorphous phase at crystal grain boundaries of the R.sub.2 Fe.sub.14 B phase, and the crystal grains of the R.sub.2 Fe.sub.14 B phase are surrounded by the R-rich phase or R-rich amorphous phase.
(iv) The easy axes of magnetization of the crystal grains in each individual magnet powder are aligned and hence the magnet powder has a magnetic anisotropy.
The magnet powder obtained by the above method (1), however, is usually crushed so as to have an average particle size of no less than 3 .mu.m, and the R.sub.2 Fe.sub.14 B phase is subjected to transgranular or intergranular fracture as shown in FIG. 1. Accordingly, the structure of the magnet powder does not become a structure wherein the crystal grains of R.sub.2 Fe.sub.14 B phase 1 are surrounded by R-rich phase 2 but become the one wherein a part of the R-rich phase 2 is allowed to adhere to a part of R.sub.2 Fe.sub.14 B phase 1, and strain caused during the crushing still remains. As a result, the prior art magnet powder by the method (1) exhibits a coercivity (iHc) of the order of only 0.5 to 3 KOe. As regards the magnet powder produced according to the method (2), when such magnet powder is employed to produce a bonded magnet, the coercivity of the resulted bonded magnet decreases with the increased molding pressure. The bonded magnet formed by pressing under a pressure of 5 tons/cm.sup.2 in an orienting magnetic field, for example, has a coercivity of no greater than 5 KOe, thereby being inferior in its magnetic properties.
In the magnet powder produced according to the method (3), the crystal orientations of the crystal grains in the R.sub.2 Fe.sub.14 B phase are arbitrary and each powder particle is isotropic in its magnetic properties. When such magnet powder is used to produce a bonded magnet, the resulted magnet exhibits a great coercivity of the order of 8 to 15 KOe. However, a great magnetic field of 20 to 45 KOe is required for magnetization since the powder is isotropic, thereby limiting its practical use.
Further, in the magnet powders produced according to the above methods, the fact that R-rich phase and R-rich amorphous phase exist at the grain boundaries of crystal grains of the R.sub.2 Fe.sub.14 B phase in such a manner as to be surrounded thereby is considered to be responsible for greater coercivities. Accordingly the existence of the grain boundary phase has reduced the percentage by volume of R.sub.2 Fe.sub.14 B phase, to thereby lower the value of magnetization of the magnetic powder.
Thus, the prior art alloy magnet powders have not made the best use of the magnetic properties which the rare earth-iron-boron alloy intrinsically possesses.